As demand from various industries for data centers grows, centers themselves have grown significantly in both size and number. One of the necessary components of any method of data center expansion is that it be scalable so that the center can continue to grow and accommodate additional workload without interruption. Point of delivery architecture, more commonly referred to as POD architecture is a scalable method of data center expansion used by Digital Realty Trust, the world’s largest landlord of data center properties. Now in its third generation, POD 3.0 uses prefabricated major electrical, mechanical, and fuel supply systems that cut down data center construction timelines and increase critical IT load capacity. Assembly time for POD components is about 20 weeks, compared to the industry average of 24 months. POD 3.0 components also boost IT capacity to 1.2 megawatts per hall, up from 1.125 megawatts in the previous generation. The improved cost point of POD design also improves energy efficiency in data centers, making Power Usage Effectiveness (PUE) ratings below 1.2 achievable.

Sources
http://www.datacenterknowledge.com/archives/2013/01/31/digital-realty-updated-pod-architecture/print/
http://c.ymcdn.com/sites/www.data-central.org/resource/collection/2DE26FAE-AE18-45D2-96B7-7B5E326EF6C9/Pod_Architecture.pdf